The invention relates to a pressure relief valve of the kind used for pressure equalization between a closed space and the surrounding atmosphere. Such pressure relief valves are e.g. used for tanks or receptacles for volatile liquids or for closed compartments for the purpose of keeping these sealed from the atmosphere under normal conditions and breaking this seal to permit pressure equalization only when the pressure within the receptacle or compartment exceeds the outside pressure by a predetermined amount.
A particularly important field of use of such pressure relief valves is for oil tankers, and in the following the invention will therefore be described with particular reference to this field of use, though it will be realized that the pressure relief valve according to the invention may also be used within other fields where similar problems exist. It is observed, also, that the pressure relief valve may, if desired, in well-known manner be combined with a vacuum valve so that pressure equalization may take place for both increasing and decreasing pressure relative to the surrounding atmosphere.
It is customary to construct a pressure relief valve of the kind referred to with a valve body which in the closed state of the valve engages an upwardly facing valve seat under the influence of its gravity, which may of course be supplemented by a spring force, if desired, though it is generally preferred to avoid springs, if possible. When the pressure below the valve seat exceeds that of the surrounding atmosphere by a certain amount, the valve body is lifted from the seat whereby gas is permitted to escape to establish pressure equalization. If such a valve seat and valve body are arranged at the top of a valve housing connected to the closed space, the gas escaping through the valve will have a tendency to spread radially outwards. The spreading of the escaping gas in relatively low height e.g. above a ship's deck may particularly in calm weather give rise to danger of fire and poisoning.
A more advantageous flow-off may be obtained by arranging an inwardly opening valve body at the top of the valve housing, but in that case the mechanical arrangement will be more complicated.